Communication system



Feb. 2,1926. 1,571,357

D. F. WHITING ET AL COMMUNICATION SYSTEM Filed Dec. 24, 1924 .9? LOCAL 5 RECEIVER I) Patented Feb. 2, 1926.

PATENT OFFICE.

UNITED, STATES DONALD F. WHITING AND HUGH S. TAYLOR, OF BROOKLYN, NEW, YORK, ASSIGNORS TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A COR- POBATION OF NEW YORK.

- COMMUNICATION SYSTEM.

Application filed December 24, 1924'. i Serial No. 757,802.

'1 all whom it may concern:

Be it known that we, DONALD F. \VHrrINo and HU H S. TAYLOR, citizens of the United States of America, residing at Brooklyn, in

the county of Kings and State of New York,

have invented certain new and useful 1mprovements in Connnunication Systems,-of which the following is a full, clear, concise, and exact description.

The invention relates to interconnnunicating systems, especially systems of the type involving operation of common receiving apparatus from different transmitting stations.

An object of the invention is to provide. a simple and effective system which will avoid confusion dueto simultaneous attempts to transmit, made at dill'erent transmitting stations.

In the embodiment of the invention specifically illustrated and described herein, operation of means to condition one of the transmitting stations for transmitting speech disable a loud speaking receiver adt jac'ent that transmitting station, in order to suppress side tone, and the operation oft means for conditioning another of the trans mitting stations for transmitting alarlnl or telegraphic code signals renders theoperated disabling means ineffective, so that the disabled receiver will be conditioned to reproduce the alarm or telegraphic code signals.

The single figure of the drawing is a circuit diagram of a system embodying this form of the invention.

The system there shown comprises telephone transmitting sets such as 1 and 2, only two of which are shown but more. of which maybe provided, receiver groups such as 3 and 4, only two of which are shown but more of which may-be provided, and an amplifier system 5 for connection between any one of the telephone transmitt-ingjsets and any desired ones of the receiver groups.

The transmitting sets 1 and 2 "are located at different places, for instance at different parts ofasbattleship on which the system may be installed.

The'receivers of groups3 and 4 are prefi erably loud speaking receivers located in different places, for instance at diflerent parts of the ship if the system is installed on shipboard.

The receiver group 3 comprises receivers such as 8 and 9, only two of which are shown though more may be provided; and the receiver group 4 comprises receivers such as 10 and 11, only two of which are shown though more may be provided. In the particular apparatus arrangement indicated in the drawing, receivers 8 and 10 are local to the telephone transmitting sets 1 and 2, respectively. --Tl1at is, the receivers 8 and 10 are intended to represent receivers which, because of their locations relative to transmitters 6 and 7, respectively, or because of other conditions of the operation of the system,

might cause objectionable side tone in transmitters'6and-7, respectively, or cause sing.- ing in the system, unless proper precautions were taken to prevent the production'of such sidev tone. Any number of such local receivers may be present in the system, though only one per group 18 shown.

The amplifier system 5 is shown as a two stage system, the first stage preferably comprising an amplifier 12 of the electron discharge type, and the second stage a pushpull amplifier 13 of the electron discharge type. 'An input transformer 14 and an input potentiometer 15 are provided for the amplifier 12, the primary winding of the transformer comprising an upper half 16 and a lower half 17. An interstage trans former 18 having a primary winding 19, and an output transformer 20 having a secondary winding 21 are provided for the amplifier system 5. The winding 21 feeds an auto-transformer 22 having output leads 23,

24 and 25 which deliver current to the re-.

symbolic of a battery, D. C. dynamo or the.

like, supplies space current for the amplifiers 1-2 and 13. A condenser 28 prevents the D. C. from source 27 from reaching the primary winding of the interstage trans electromotive force 26,

former 19. Inseries with the primary winding of transformer 19 is the primary winding 29 of a feed back transformer 30, which has a secondary winding 31. Means are provided, as described hereinafter, for connecting the winding 31 to the primary winding of the transformer 14, to convert the amplifier 12 into a feed back oscillator, for a purpose set forth hereinafter, a condenser 32 being connected across the winding 31 for setting the frequencyof oscillation. The winding 29 is normally short circuited by a contact 33 of a normally deenergized relay 34; and the winding 31 is normally short circuited by a contact 35 of the relay 34. v c

When the amplifier 12 is to be converted into an oscillator .as' mentioned above, the

relay 34 is energized as described hereinafter, to open its short circuit-ing contacts, and a relay 36 is also energized as described hereinafter. The relay 36 has an armature 37 for changing the setting of the input potentiometer 15, and also has ,an armature 38. Armature 38 normally closes a gap in .a circuit traced hereinafter for supplying D. C. from source 26 to conductors 39 and 40.- By the operation of switches as described hereinafter, conductors 39 and 40' together, can be selectively connected to transmitters 6 and 7, for delivering energizsmg D. C. thereto, and for delivering A. G.

' from the selected transmitter to the primary traced hereinafter. gized, armature 38 of armature 38,

winding of transformer 14 over a circuit When relay 36 is eneropens its normal contact, and closes itsright hand contact to connect the condenser 32 and the secondary winding 31 of the feed back transformer 30 to the primary winding of the input transformer 14, the circuit from condenser 32 and winding 31 extending from the lower side of the" condenser and the winding through a conductor 41, the upper half 16' of the primary winding of transformer 14, a part of condnctor 39, a. conductor 42, right hand contact aconductor 43, the lower half 17 of the former 14, a condenser 44, and a conductor 45 to the upper side of condenser 32 and contacts operable toconnect transmitter 6 to.

conductor '39, to complete the connection of the transmitter to the conductors 39 and '40.

primary winding of trans-- Similarly, switch 48 comprised in telephone transmitting set 2 and has a pair of inner left hand alternate contacts operable to connect transmitter conductors 39 .and 40.

A normall deenergized receiver group connecting re ay, such as relay 49 and relay 50, is provided for each receiver group such as 3 and 4. The relay 49 when operated connects conductors 23 and 24 to conductors 51 and 52, respectively, which lead to receiver group Similarly the relay 50, when operated, connects conductors 23 and 25 to conductors 53 and 54, respectively, which lead to .receiver group 4.

Asource'of D. C., 55, symbolic of means comprising a storage battery, D. C. dynamo or the like, energizes relay 49 Whenever the I outer right hand contacts of switch 47 and the contacts of a receiver group selecting switch 56in series-therewith are closed, or

whenever the outer right hand contacts, of

switch 48 and the contacts of a receiver group selecting switch 57 in series therewith are closed. The switches 56 and 57 are comprised in the telephone transmitting sets 1 and 2, respectively. The circuit for thus energizing relay 49 through switches 47 and 56 extends from the upper terminal of source 55 through the winding of relay 49, conductors 58 and 59, switch 56, outer right hand contacts of switch 47, and conductors 60 and 61 to the lower terminal of source 55. Similarly, the circuit for energizing relay 49 through switches 48 and 57 extends from the upper terminal of source 55 through the winding of relay 49, conductors 58 and 62, switch. 57, outer right hand contacts of switch 48, and conductors 60 and 61 to they lower terminal of. source 55. ,Thus, switch 56 and the contacts of switch 47 that are in series therewith arein parallel with switch 57 and the contacts of switch 48 that are in series therewith.

The source 55 energizes relay 50 whenever the inner right hand contacts of switch 47 and. the contacts of a receiver group selecting switch-64 in series therewith are closed,

.or whenever the inner right hand contacts of switch 48 and the contacts of a receiver group selecting switch 65 in series therewith are closed. The switches 64 and 65 are comprised in the telephone transmitting sets 1' and 2, respectively. The circuit for thus energizing relay 50 through switches 48 and 64 extends from the upper terminal of source 55 through the winding of relay 50, leads 66- and-67, switch 64, in'ner'right hand contacts of switch 47, and leads 60 and 61 to the lower terminal of source 55. Similarly, the circuit for energizing relay 50 through switches 48 and 65 extends from the upper terminal of source 55 through the winding of relay 50, conductors 66 and 67, switch 65,

inner right hand contacts of switch 48, and conductors 60 and 61 to the lower terminal of source 55'. Thus, switch 64 and the contacts of switch 47 that are in series therewith are in parallel with switch 65 and the contacts of switch 48 that are in series therewith.

From the foregoing description it will be clear that from any telephone transmitting station, such as stations 1 and 2, any desired ones of the receiver groups such as 3 and 4 can be connected to auto-transformer 22.

With the system in the condition in which it is shown, if switch 47 be operated from the position in which it is shown into its alternate position, its inner left hand contacts will connect telephone transmitter 6 to conductors 39 and 40 as noted above, and consequently the transmitter will be energized by direct current from source 26 and will be operatively connected to transmit telephonic currents to the primary winding contacts of switch 47 to conductor 39,

through winding 16, conductor 41, contacts 35 of relay 34, conductor 45, condenser 44,

winding 17, conductor 43, left hand contact of armature 38, and lead 40, to the transmitter 6. i l

The telephonic currents thus transmitted to the primary winding of transformer 14 will be amplified by the amplifier system 5 and the amplified current will be delivered to auto-transformer 22. (Since contacts 33 and 35 short circuitthe primary and second ary windings, respectively, of the feed-back transformer 30, the feed-back path is ineffective. under the conditions assumed.) Therefore, if any one of the receiver group selecting switches, such as 56 and 64, in the telephone transmitting set 1 to operated, the corresponding receiver groups, such as 3 and 4, respectively, will receive the ampli-- fied telephonic current from auto-transformer 22. However the receiver 8 will be cut out of circuit by the outer left hand contacts of switch 47, which maintains a gap in the conductor 52 at a point between receiver 9 and receiver 8 whenever switch 47 is in the conditionalternate to that in which it is shown}.. Y 1 v This disabling of receiver 8 when transmitter 6 is conditioned to operate other re-' ceivers, such as receiver 9 of group 3 and receivers 10 and 11 of group 4, will be desirable in case, for instance, receiver 8 if it were not disabled would be more likely than such other receivers to produce objectionable side tone in transmitter 6, for example because of closer proximity of receiver 8 than such other receiver to transmitter 6.

Vith the system in the condition in which it is shown, if switch 48 be operated from the position in which it is shown to its alternate position, its inner left hand contacts will connect telephone transmitter 7 to conductors 39 and 40 as noted above, and con sequently the transmitter will beenergized by direct current from source 26 and will be operatively connected to transmit-telephoniecurrent to the primary winding of transformer 14. The D. C. energizing cir= cuit for the transmitter 7 and the circuit for transmitting telephonic current from the transmitter 7.to the'primary winding of transformer. 14 will, be the same as the corresponding circuits traced just above for transmitter 6, except that transmitter 7 and i the inner left hand contacts of switch 48 will be substituted for the transmitter6 and the inner left hand contacts of switch 47. The telephonic current thus transmitted from transmitter 7 will be amplified by the amplifier system 5. Therefore, if any oneof the receiver. group selecting switches, such as 57 and 65, in the telephone transmitting set 2 be operated, the corresponding receiver groups, such as 3 and 4, respectively, will receive the amplified telephonic current from auto-transformer 22. However, the receiver 10 will be cut out of circuit by the outer left hand contacts of switch 48, which main-' tain a gap in the conductor 53 at a point between receiverll and receiver 10 whenever switch 48 is in the'eondition alternate to that in which it is shown. The pu'rposein thus disabling receiver 10 is to prevent it from causingobjectionable side tone in transmitter 7.

. Alarm signal transmitting sets such as 71 and 72 are provided, comprising switches 73 and 74, respectively, and switches 75 and 76, respectively. The alarm signal transmitting sets may have any desired locations such as 1 and 2., but preferably each telerelative to the telephone transmitting sets phone transmitting set has an ,alarm signal transmitting set individual thereto and 10- cated thereat. Thus, the drawing indicates set 71 as adjacent "set 1, and indicates set 72' as adjacent set 2. v

To transmit alarm signals from set- 71, switch'73 is operated from the condition in which it is shown tothe alternate condition, and switch 5 is closed, these operations causing the receivers such as 8, 9, 10 and ll tacts of switch 7 3 are to emit a tone, as described hereinafter. Similarly, to transmit alarm signals from set 72, switches 74 and 76 are operated, causmg the receivers to emit a tone, as described hereinafter. \Vhen switch 73 is operated, the outer right hand contacts thereof, which are in parallel with the series circuit comprising switch 56 and the outer right hand contacts of switch 47, close an energizing circuit for relay 49, this energizing circuit being the same as that traced above for energizing rclay 49 through the series circuit justmentioned, except that .the outer right hand consubstituted for that series circuit. 7'

Similarly, when switch 73 is operated, the inneinright hand contacts thereof, which are in parallel with the series circuit comprising switch 64 and the inner right hand contacts of switch 47, close an energizing circuit for relay 50, this .energizing circuit being-the same as that traced above for energizing relay 50 through the series circuit just mentioned, except that the inner right hand contacts of switch 73 are substituted for that se-- ries circuit.

The outer and inner right hand contacts of switch 74 are in parallel with the outer and inner right hand 'contacts, respectively, of switch 73; so the energization of relays 49 and 50 can be controlled by switch 74 just as by switch 73.

From the foregoing description it will be.

apparent that whenever a switch such as switch 73 or switch 74-1n any alarm signal transmittingset such as 71 or 72 is operated,

all of the receiver groups, such as 3 and 4, will be connected to the auto-transformer 22 by the receiver group conecting relays'such as 49 -and -50.

lVhenever switch 73 is operated, a circuit for energizing relay 36 is closed by the outer left hand contacts of switch-73, this eniergizing circuit extending from the upper terminal of source 55, through conductor 77, winding of relay '36; conductor 78, outer left hand contacts of switch 73, and conductor 61 to the lower terminal of source 55. Relay 36 will also be operated whenever switch 74 is operated, since the series circuit consisting of conductor 79, outer left hand contactof switch 74 and conductor 63 is in parallel with that part of the circuit just traced, consisting of conductor 78, outer left hand ,contactof switch 73 and conductor 60. Therefore it will be seen that relay 36 is energized whenu ever a switch such as switch 73 or switch 74 in any alarm signal transmitting setsuch as 71 or 72 isv operated. By closing a switch such as 75 or 76 in any alarm signal transmitting set such as 71 or 72, the relay 34 can be energized. The energizin circuit when switch 75' is closedextends from the upper terminal of source 55, through the winding of relay 34, conductor 81, switch 7 5, and conductor 61 to the lower terminal of source 55; and the energizing circuit closed by switch 76 is the same, except that for the circuit consisting of conductor 81, switch 75 and conductor 61 in series, there is substitut ed the parallel circuit consisting of conductor 82, switch 76 and conductor 63 in series.

As described above, the energization of relays 36 and 34 as just mentioned converts the amplifier into an oscillator, disconnects the transformer 14 and source 26 from conductors 39 and 40, changes thesetting of the potentiometer 15 and removes the short circuits from the feed-back path through transformer 30, so that the amplifier 1.2 is converted into an oscillator. Output current from the oscillator is amplified by amplifier 13 and delivered therefrom to the circuit comprising transformer 20, auto-transformer 22 and the receiver groups such as 3 and 4.

If the transmission of such alarm signals is initiated at an alarm signal transmitting set remote from a given telephone transmitting set when that telephone transmitting set is in use by an operator, that operator may be unaware that his transmitter has been disabled (disconnected from transformer 14 and source 26) unless some provision is made to call to his attention the disabling ofhis transmitter; for when he operated his switch such as 47 or 48 he disabled any receiver local thereto in order to avoid producing objectionable side tone in his transmitter, as described above. Therefore, a relay 83 is provided for restoring all of the disabled receivers whenever the system is operated to produce alarm signals.

Whenthe relay is energized as described hereinafter its right hand contact closes a circuit extending from the left hand armature of relay. 49-, through the right hand contacts of relay 83, and conductor 84, to a point in conductor 52 lying between the receiver 8 and the outer left hand contact of extending from the. right hand armature of relay 50, through'the left hand contactsof relay 83, and conductor 85, to a po1n t In conductor 53 lying between the receiver 10 and the outer left hand contact of, switch 48. This circuit is in parallel with the part of conductor 53 lying between the point ust referred to and the right hand armature of relay 50, and therefore thegap in conductor receivers 10 and 11 willbe operated in paral-- lel in spite of the open normal contact of switch 48.

The energization of relay 83 referred to above is effected when a switch such as.73

or "74 in any alarm signal transmitting set is operated. The circuit closed by switch 73 for energizing relay 83 extends from the upper terminal of source 55, throughthe winding of relay 83, conductors 86 and 87, inner left hand contacts of switch 73, and conductor 61 to the lower terminal of source 55. The circuit which switch .74 can close, for energizing rela 83, is the same as that "just traced througswitch 73, except that the path consisting of conductor 88, inner left hand; contacts of switch 74, and conductor 63, and connected in parallel to the path consisting of conductor 87, inner left hand contacts of switch 73, and conductor 60, is substituted for the latter path. When an alarm tone is being sent from an alarm signal transmitting 'set, such as'71 or 72, the tone may be broken up to form, for

instance, telegraphic code signals, by open ing and closing the switch such as 75 or 76 at the station in use, in the general fashionin which a' telegraph key is customarily operated to transmit the dots 1 and dashes forming telegraphic code characters. 1

A frequency of about 1200 cycles per sec- -ond has been found satisfactory as the oscillation frequency of the amplifier 12 when that amplifier is being employed 'as an oscillator;' but of course the oscillation frequency may be set at any value suitable for the operation of the receivers.

7 As mentioned above,the settingof potentiometer 15 is controlled by the armature of relay 37.

it is, desired to maintain during telephonic transmission, by proper adjustment of the f By means of the tap conductors 24 and 25,-

the connections of 'receiver'groups to the auto-transformer 22 can be independently adjusted to give the most favorable operating conditions for transmission.

: In operating the system to transmit speech.

from, i's'ay, transmitter 6 y to receiver groups 3 -a nd-,4, '.operator at transmitti'ngf'set 1 closes switches56, 64 and 47 and then talks Iinto transmitter. 6. The closure of switch 56 and the. operation of switch 47 cause relay 49 to be energized over a circuit extending from the upper terminal of source .55,

- through the winding of relay 49, conductors .58 and-59, switch 56,.oute'r' right-hand con- .tacts ofswitch 47 and conductors 60 and 61, tothe lowerterminal of source 55. Conse- This feature renders it possible to set the amplifier gain at the .value which.

quently, relay 49 connects receiver group 3 'ing' any local receiver such as 8, conductor 51, right-hand contact of relay 49, and conductor 23 to auto-transformer 22. Any receivers of receiver group 3, which are local to transmitter 6 (such as receiver 8), will be omitted from the circuit just traced because the operation of switch 47 has opened conductor 52 at the outer left-handnormal contact of'this switch.

The closure of switch 64 and operation of switch 47, as mentioned above, cause relay 50 to be -energized over a circuit extending from the upper terminal of source 55, through winding of relay 50, conductors 66 and 67, switch 64, inner right-hand contacts of switch 47, and conductors 60 and I 61 to the lower terminal of source 55. Confrom tip conductor 25, through the lefthand contacts of relay 50, conductor 54, receivers 10 and 11 in parallel, conductor 53, the right-hand contacts of relay 50, and conductor 23 to the auto-transformer 22.

The operation of switch 47 as mentioned above causes transmitter 6 to be energized over a circuit extending from the positive terminal of source 26, through conductors "69 and 45, contacts 35, conductor 41, winding 16, conductor 39, inner' left-hand con- 'tacts of switch 47, transmitter 6, conductor 40, left-hand contact of armature 38, conductor 43, winding 17, and conductor 70 to the negative terminal of source 26.

mitter 6 energize the primary winding of input transformer 14, through a circuit extending from the upper terminal of transmitter 6, through the inner left-hand contactsv of switch 47, conductor 39, winding 16,

conductor 41, contacts 35, conductor 45, condenser 44, winding 17 conductor 43, lefthaIidcontact ofarmature 38, and conductor .40 to the lower terminal of transmitter 16. Speech currents induced in the secondary winding of transformer 14 pass through potentiometer 15 and cause voltage variations between the grid and filament of amplifier 12, the filament of amplifier 12 being connected to potentiometer 15 through the right h'and F'contact of armature 37. Amplified speech-,currents traverse the circuit extendin from. the anode of, amplifier 12, through condenser 28, primary winding 19 of interstage transformer 18, and contacts 33-to' the filament ofampli'fier 12. The speech currents induced in the secondary winding of transformer 18. are amplified by repeater 13 and delivered through trans- The speech currents generated by transceivers 9, 10 and 11.

former 20 to auto-transformer 22, and thence over the circuits traced above to re- VVhen the operator at transmitter 6 has finished transmitting his message, he releases switch 47 ,which returns to its normal position, thereby restoring the system to its normal condition shown in the drawing, vexcepting that switches 56 and 64 remain closed. He may open these switches at the termination of his message, or may allow them to remain closed until occasion arises for him to transmit to one of the receiver groups 3 or 4 to the exclusion of the other group, when he will open the switch 56 or 64 corresponding to the groupswhich is to remain idle.

If occasion arises for transmission of alarm signals from alarm signal transmitting set 72 at a time when an operator is transmitting through transmitter 6, the system is operated in the manner now to be described. The operator at set 72 closes switch 74. The closure of the inner and outer righthand contacts of switch 74 shunts the inner and outer contacts, respectively, of switch 73, so that receiver groups 3 and 4 become or remain connected to auto-transformer 22 regardless of whether the operator at set 1 allows switches 56 and '64 to remain closed. Moreover, the closure of the inner left-hand contact of switch 74 causes energization of relay 83 over a circuit extending from the upper terminal of source 55, through the winding of relay 83, conductors 86 and 88, innerleft-hand contacts of switch 74, and

conductors 63 and 61 to the lower terminal of source 55. Consequently, the right-hand contacts of relay 83 shunt the outer left hand contacts of switch 47, so that the latter contacts, even if opened, will be inelfective to disconnect receiver 8 from autotransformer 22.

The closure of the outer left-hand contact of switch 74 causes energization of relay 36 through a circuit extending from the upper terminal of source 55, through conductor 77,'winding of relay 36, conductor 79, outer left-hand contacts of switch 74 and conductors .63 and 61 to the lower terminal of source 55. Consequently, armatures 37 and 38 operate to their inner contacts The operation of armature 37 changes thesetting of potentiometer 15 from tap conductor 89 to tap conductor 90. The operation of armature 38 breaks the D. O. energizing circuit traced above for transmitter 6; This break is shunted by condenser 46, as described above. The operation of armature 38 also breaks the talking circuit, traced above, etween transmitter 6 and transformer 14. The closing of the'inner contact of armature 38 also closes a gap in a circuit to be traced presently. The operator at set 72 then closes switch 76 while maintaining switch 74 closed. The closure of switch 76 energizes relay 34 over a circuit extending from the upper terminal of source 55, through conductor 77, winding of relay-34, conductor 82, switch 76, and conductors 63 and 61, to the lower terminal of source 55. Consequently, relay 34 opens the short-circuiting contacts 33 and The amplifier 12 now oscillates, generating signaling currents, for instance of a frequency of 1200 cycles per second, which are delivered through amplifier l3 and transformer 20 to auto-transformer 22, and thence to all of the receivers 8, 9, 10 and 11. The output circuit of the oscillator extends from the anode through condenser 28, winding 19, and winding 29 tothe filament. The input circuit of the oscillator extends from the upper side of condenser 32 and winding 31, through conductor 45, condenser 44, winding 17 ,-conductor 43-, inner contact of armature 38, conductor 42, winding 16, and conductor 41 to the lower side of condenser 32 and winding 31. By opening and closing switch 76 while maintaining switch 74 closed, the operator at set 72 can cause the oscillator to deliver to the receivers a tone intarrupted in accordance with a telegraphic co e.

When the operator at station 72 releases switches 76 and 74, the system returns to its normal condition as shown in the drawing.

Irom the foregoing description of operatlons of the system to perform certain functions, the operation of the system to perform lowing claims.

What is claimed is:

1. A system comprising a receiver, means for transmitting signals,switching means operable'for preventing signals transmitted by said first means from returning to said first means through said receiver, and means for causing signals to reach said first means through said receiver, while said second means is in its operated condition.

3. A communication system comprising a tele hone transmitter, two receivers normal v so connected together as to operate together, means-operable to cause connection of said transmitter'to one of said receivers while the other receiver remains disconnected from-said transmitter, a signal transmitting station, and means for operating said other receiver from said signal transmitting both of said receivers from said signal transmitting station while said first mentioned means is in its operated condition and both receivers are disconnected from said transniitter.

5. A telephone and alarm signal system comprising two telephone transmitters, two receivers normally so connected together as to operate together, means operable to cause connection of one of said transmitters to one of said receivers while the other of said receivers remains disconnected from said one transmitter, means operable to connect said other transmitter to both of said receivers at once, an alarm signal transmitting sta tion, and means for operating said otherreceiver from said alarm signal transmitting station'while said first mentioned means is in its operated condition,

6. A telephone and alarm signal system comprising two telephone transmitters remote from each other, a group of receivers, means operable to cause selective connection of said transmitters to said receiver group, means operable in response to a given operation of said first means to cause. one of said receivers to remain disconnected from the transmitter selected by said given operation of said first means while another of said receivers is connected to said transmitter by said .given operation of said first means, an alarm signal transmitter station, and means for operating both of said receivers from said alarm signal transmitting station while said first means is in its operated condition.

7. A system comprising an amplifier hav ing an input circuit and an output circuit a feed back path for connecting said output circuit to said input circuit to cause said amplifier to oscillate, and means for shortcircuiting said feed-back path and prevent ing longitudinal transmission of waves from said output circuit to said 1 input circuit through said path,

8. A system comprising a signalling transmitter, an amplifier, said amplifier having an input circuit and an output circuit with a feed-back transformer having a primary winding and-a secondary winding for connecting said circuits, and said input circuit having a. gain adjusting potentiometer, means normally short circuiting said primary winding, means normally short circuiting said secondary winding, and means for causing change in the setting of said potentiometer, and for causing said short circuits to be open while said. potentiometer is operating in its altered setting, to convert said amplifier into an oscillator for gener ating signalling currents.

9. A system comprising a signaling transmitter, an amplifier, having an input circuit and an output circuit with a common terminal, for repeating signals from said transmitter, a feed-back path connected inseries with said output circuit, for'connecting said output circuit to said input circuit to convert said ,amplifier into an oscillator, said feed-back path having one end so connected to said common terminal as to be maintained at the potential of said common terminal while said amplifier is repeating said signals from said transmitter, and means for short circuiting said feed-back path.

10. A system comprising a signaling transmitter, an amplifier, having an input circuit and an output circuit, for repeating signals from said transmitter, said input circuit including a transformer having a primary winding and a secondary winding, a

feed-back path for connecting said output circuit to energize said primary winding, for converting said amplifier into an oscillator, and means for short-circuiting said feedback path and preventingtransfer of waves from said output circuit to said secondary winding through the electrostatic capacities between said windings. 1

In witness whereof we hereunto subscribe our names this 23rd day of December A. D., 1924.

DONALD F. WHITING. HUGH S. TAYLOR. 

